1. Field of the invention
The present invention relates to a voltage non-linear resistance composition used to protect the semiconductor or electrical circuit from the surge or noise; and voltage non-linear resistance element using thereof.
2. Description of the Related Art
Recently, electrical circuits made of semiconductor, LSI and etc has advanced in high performance; and it has been used in many purposes and environments. However, in many cases, these semiconductors and electrical circuits work at low voltage, and if excessive voltage is applied, these were liable to be destroyed. Especially, due to abnormal surge voltage and noise such as lightning, the electrostatic is discharged and the voltage thereof will be applied to the semiconductor element or so and it can be destroyed. These problems are particularly prominent in portable devices used in various environments.
In order to overcome such situations, protective element is set in parallel connection to the semiconductor element in many cases. This protective element has large resistance when normal voltage is applied to the above semiconductor element, thus the current will flow mainly to the above semiconductor element allowing this semiconductor element to run properly. On the other hand, when excessive voltage is applied, the resistance of this protective element will decline. Due to this, the current will flow mainly to the protective element suppressing excessive current to flow into this semiconductor element. Therefore, this semiconductor element is protected from being destroyed by the flow of excessive current.
The current-voltage characteristics of these protective elements must have non-linear characteristics. That is, the changes depending on the voltage, for example, it has characteristics such as the dramatic decline of the resistance at above certain voltage. Zener diode and varistor (voltage non-linear resistance element) is known as an element obtaining such characteristics. Compared to the zener diode, varistor has no polarity in the movement, has higher surge resistance, and is easier to make it compact, hence it is specially preferred to be used.
As for the varistor, various materials (voltage non-linear resistance ceramic composition) are used, however particularly the sintered body having the ZnO as the main component is preferably used due to the cost and the size of the non-linearity (for example, Japanese Patent No. 3493384 and Japanese Unexamined Publication No. 2002-246207). An example of current-voltage (logarithm) characteristics is shown in FIG. 10. The resistance significantly declines at voltage larger than the breakdown region, and the current becomes larger. The voltage (V1 mA) which makes the current 1 mA is called varistor voltage, and when the voltage exceeds thereof, large current will flow. The varistor voltage is higher than the voltage which the semiconductor works properly (for example 3V or so), plus varistor voltage is set accordingly to the voltage which the difference from this voltage is not too big.
In these voltage non-linear resistance ceramic compositions, the main component is set to ZnO; and as impurities to give conductivity and non-linearity of current-voltage or so to this, Pr (rare earth element), Co, Al (IIIb group element), K (Ia group element), Cr, Ca, and Si are added. By controlling these concentration, improvements in varistor lifetime (Japanese Patent No. 3493384), and lowering of non-uniform production of varistor (Japanese Unexamined Publication No. 2002-246207) are accomplished.